Mobile robotic vehicle

ABSTRACT

A mobile robot includes a robot chassis having a forward end, a rearward end and a center of gravity. The robot includes a driven support surface to propel the robot and first articulated arm rotatable about an axis located rearward of the center of gravity of the robot chassis. The arm is pivotable to trail the robot, rotate in a first direction to raise the rearward end of the robot chassis while the driven support surface propels the chassis forward in surmounting an obstacle, and to rotate in a second opposite direction to extend forward beyond the center of gravity of the robot chassis to raise the forward end of the robot chassis and invert the robot endwise.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation (and claims the benefit of priorityunder 35 USC 120) of U.S. application Ser. No. 12/331,380, filed Dec. 9,2008 now U.S. Pat. No. 7,926,598. The disclosure of the priorapplication is considered part of (and is incorporated by reference in)the disclosure of this application.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

This invention was made in part with Government support under contract#FA8650-08-C-7815 entitled “Ember: a Small, Inexpensive, and SmartMobile Communications Relay Platform” awarded by the DARPA IPTOLANdroids program. The Government may have certain rights in theinvention.

BACKGROUND

The invention relates generally to robotic mobile platforms.

Robots are useful in a variety of civilian, military, and lawenforcement applications. For instance, a robotically controlledmobility platform can be used to inspect or search buildings underhazardous or hostile conditions. Dangerous situations can be improved byproviding detailed information about the location, activities, andcapabilities of opponents. Military applications can includereconnaissance, surveillance, bomb disposal and security patrols.

Advances are sought in the miniaturization of robots and the ability ofrobots to surmount obstacles.

SUMMARY

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

One aspect of the invention features a mobile robot including a robotchassis having a forward end, a rearward end and a center of gravity.The robot further includes a driven support surface connected to thechassis and configured to propel the robot chassis forward and rearward.A first articulated arm is rotatable about an axis located rearward ofthe center of gravity of the robot chassis and is configured to: trailthe robot, rotate in a first direction, raise the rearward end of therobot chassis while the driven support surface propels the chassisforward in surmounting an obstacle, and rotate in a second oppositedirection to extend forward beyond the center of gravity of the robotchassis to raise the forward end of the robot chassis and invert therobot endwise.

In some embodiments, the driven support surface includes a flexibletrack trained about a pair of wheels.

In some embodiments, the axis of the arm is coaxial with an axis of aone of the wheels.

In some embodiments, the robot includes a second articulated armrotatable about the axis with the first articulated arm.

In some embodiments, the first and second arms are located outward ofthe driven support surface and are continuously rotatable in eitherdirection. In some embodiments, an axle connects the first and secondarms and an idler wheel is freely rotatable about the axle.

In some embodiments, the first arm is located substantially along acentral longitudinal axis of the robotic chassis. In some cases, thefirst arm is configured with a camera.

In some embodiments, the first arm is continuously rotatable to providea swimming-type propulsion.

In some embodiments, the first arm is configured to rotate as a functionof an angle of incline of the robot chassis.

In some embodiments, a duration of rotation of the first arm ispredetermined as a function of an angle of incline of the robot chassis.

In some embodiments, the robot includes a radio transceiver and thefirst arm is rotatable to raise the robot chassis to elevate thetransceiver. In some cases, the robot is a mobile mesh network noderobot.

In some embodiments, the robot includes a cliff detector at the forwardend and a proximity sensor on a side of the robot. In some cases, therobot is configured to maintain a fixed proximity to a detectedobstacle. In some cases, the fixed proximity is maintained by comparingreadings from the first proximity sensor and a second proximity sensorand turning the vehicle to substantially equilibrate the readings.

In some embodiments, the robot includes an angular rate sensor.configured to detect an impact of the robot with an obstacle and anangle of incline of the robot.

In some embodiments, the robot includes a slip clutch between the firstarm and a first arm drive motor.

In some embodiments, a pull pin is removably received by the chassis. Insome cases, the pull pin locks the first arm in a stowed position. Insome cases, the pull pin immobilizes the first arm. In some cases, thepull pin is configured to power-on the vehicle upon removal from thechassis. In some cases, the pull pin is configured to release an antennafrom a stowed position and to unlock the first arm from a stowedposition.

In some embodiments, the vehicle substantially fits within a boundingvolume approximately 7 inches long, 5 inches wide and 2 inches tall.

In some embodiments, the first arm is rotatable to align with ahorizontal axis of the vehicle. In some cases, the vehicle is configuredto fit in a combat uniform cargo pants pocket.

Another aspect of the invention features a method for operating a mobilerobotic vehicle having a driven support surface and a first pivotingtrailing arm to surmount obstacles including a series of stair risers.The method includes driving the support surface to propel the vehicle tocontact the riser of a first stair with a forward end of the vehicle,and driving the support surface to cause the forward end of the vehicleto ascend the riser of the first stair. The method further includespivoting the first arm to raise a rearward end of the vehicle as theforward end of the robot approaches the top of the riser of the firststair; driving the support surface to advance the forward end of therobot over the top of the stair riser; and pivoting the arm to furtherraise the rearward end of the vehicle such that the forward end of thevehicle rotates downward beyond the top of the riser of the first stair.

In some applications, the method includes repeating the driving andpivoting to surmount a second stair.

In some applications, the method includes pivoting the arm to raise theforward end of the vehicle above an underlying surface.

In some applications, the method includes further pivoting the arm toflip the robot endwise.

In some applications, raising the forward end of the vehicle raises aradio transceiver on the vehicle.

In some applications, the method includes supporting the vehicle in afirst substantially horizontal orientation on an underlying surface,pivoting the arm to contact the underlying surface to raise the forwardend of the vehicle and further pivoting the arm to rotate the forwardend of the vehicle past a stable point and allowing the vehicle totopple over to a second orientation inverted with respect to the firstorientation.

In some applications, the first arm is pivoted to raise the rear end ofthe chassis to maintain a chassis angle of incline of less than about 45degrees as the forward end of the chassis surmounts a stair riser.

In some cases, the stair risers comprises spaced apart obstaclesincluding a rise in elevation.

In some applications, pivoting the first arm and driving the supportsurface are performed simultaneously. In some applications, the pivotingthe first arm and driving the support surface are performedsubstantially asynchronously.

In some applications, the support surface propels the vehicle to apredetermined angle of incline before the first arm is initially pivotedto raise the rearward end of the vehicle.

In some applications, the support surface is further driven while thefirst arm is further pivoted to surmount the riser.

In some applications, the first arm is pivoted at a predetermined ratefor a predetermined period upon detection of a predetermined angle ofincline of the vehicle.

In some applications, the first arm is pivotally retracted to a pointabove the bottom of the driven support surface upon the vehiclesurmounting the riser.

In some applications, the first arm is pivotally retracted to apredetermined angle to provide clearance for the vehicle to ascend asecond riser to a predetermined angle of incline before the first armcontacts an underlying surface.

In some applications, the method includes detecting when the robot hitsan obstacle and generating a random recoil turn rate command.

In some applications, the method includes propelling the vehiclebackwards at a preset speed while turning at the random recoil turn ratefor a fixed period of time or until the vehicle hits another obstaclemoving backwards.

In some applications, the method includes, upon detection of impact ofthe vehicle with an obstacle while moving backwards, generating a secondrandom recoil turn rate and propelling the vehicle forward at a presetspeed while turning at the second turn rate.

In some applications, the method includes detecting an obstacle at adistance and turning the vehicle to avoid the vehicle.

In some applications, the method includes propelling the vehicle in aspiral pattern at a preset speed and a preset diminishing turn rateuntil an obstacle is detected.

Another aspect of the invention features a robot including a body and adifferential drive supporting the body. The differential drive includesa left drive motor that turns a left drive wheel and a right drive motorthat turns a right drive wheel, each of the left drive wheels and rightdrive wheel are turned about a common first axis. A flipper arm includesa pivot end and a distal end, the flipper arm being supported withrespect to the body to pivot about a second axis parallel to the commondrive axis to revolve the distal end about the second axis. A flipperdrive, including a flipper drive motor, is connected to the flipper armto drive the flipper arm through a continuous 360 degrees of revolutionabout the pivot axis. A left motor circuit controls the left motor, aright motor circuit controls the right motor and a flipper motor circuitcontrols the flipper motor. A body attitude sensor measures tilt of thebody from the direction of gravity, about a third axis parallel to thefirst axis. A flipper angle sensor measures angular position of theflipper about the second axis. A microcontroller commands the left motorcircuit, right motor circuit, and flipper motor circuit and includes anobstacle climbing routine that monitors the body attitude sensor and theflipper angle sensor. The obstacle climbing routine commands the flippermotor circuit to (a) move the flipper to revolve to a positionsubstantially extending along the ground as the differential driveclimbs a face of an obstacle, and (b) move the flipper to revolvebetween the position extending along the ground to a position extendingbelow the body as the differential drive drives forward and overcomes atop of the face of the obstacle, thereby tipping the body over the topof the face of the obstacle.

In some cases, the flipper is moved to extend along the ground when thebody passes more than a certain acute angle, e.g., 45 degrees, fromhorizontal. For example, the flipper is moved to be positioned about thesame acute angle from a direction normal to the top of the body.

In some cases, the body includes a frame chassis, monocoque or unibodyhull.

Another aspect of the invention features a method of robot obstacleclimbing. The method includes monitoring an attitude of a robot bodyhaving a differential drive, monitoring an angle of a pivoting flipperwith respect to the robot body; pivoting the flipper to a positionextending along the ground as the differential drive climbs a face of anobstacle. The flipper is pivoted from the position extending along theground to a position extending below the body as the differential drivedrives forward and overcomes a top of the face of the obstacle, in orderto tip the body over the top of the face of the obstacle as the distalend of the flipper supports the body below the top of the face of theobstacle.

Another aspect of the invention features a ground robot including asubstantially box-like rectangular body no more than two inches inheight, ten inches in length, and ten inches in width, and having aleading end and a trailing end, and having no more than 1 kg mass. Adifferential drive supports the body and includes a left driven trackand a right driven track each no less than ½ inch in width,differentially driven about a drive axis. A flipper arm having a pivotend and a distal end is supported at its pivot end from the body no morethan three inches from the trailing end of the body to pivot about aflipper axis parallel to the drive axis. The flipper arm is rotatablethroughout a continuous 360 degrees of revolution about the pivot axis.

In some cases the differential drive includes tracks with sufficienttraction to climb the leading end of an obstacle composed of commonmaterials such as wood, asphalt and concrete by driving the leading endof the tracks up a face of an obstacle while the trailing end tracksdrives along the ground.

In some applications, the robot is configured to prevent the robot fromsliding back down the obstacle when the leading end of the track clearsthe top of the obstacle, and to hold and advance the robot at the top ofthe obstacle as the trailing end of the tracks are lifted off theground.

In some applications, the differential drive propels the robot forwardto drag the distal end of the flipper arms along the ground as theflippers are rotated in to swing the distal ends of the flippers in thesame direction as the robot is advancing.

In some cases, the flippers are less than 3% of the total weight of therobot and do not appreciably move the center of gravity of the robotduring rotation of the flippers. In some cases, the distal ends of theflippers are sized to partially penetrate into certain loose terrainssuch as snow, sand and gravel. The flippers are substantially rigid toresist bending.

In some case the distal ends of the flippers are rounded or tapered toslide along normal surfaces such as wood, asphalt, and concrete as therobot advances over the top of an obstacle using just the traction ofthe leading ends of the tracks.

In some cases the distal ends of the flippers are rounded to permit thedistal end of the flipper to slip through and past a bottom dead centerposition in order to push the trailing end of the robot as high aspossible before the robot is finally driven to overcome the obstacle.

In some embodiments, the robot is sized to be readily portable, and tosubstantially fit within a bounding volume approximately 18 cm (7 in.)long, 12 cm (5 inches) wide and 5 cm (2 inches) tall. A trailingpivoting arm allows the compact robot to climb obstacles as big asitself, including stairs. A particular small robot embodiment is about13 cm (5″) axle to axle, 3 cm (1″) diameter wheels with a 15 cm (6″)overall length, capable of climbing about obstacles up to about 13 cm(5″). Such compact embodiments can be carried in the various pockets andpoaches contained in the law enforcement or militia uniforms. Anotherembodiment is scaled large enough to climb standard stairs 17-20 cm(7″-8″). These robots are well-suited to urban settings such as onrooftops, sidewalks, stair wells, streets and in indoor residential andoffice environments.

In use, the robot is removed from a pocket, a deactivation plug or pinis removed from the side of the robot and the robot is placed or eventossed in a suitable location. This process can be repeated to create amulti-node mesh communications network. Multiple robots can act as radiorelays, forming multi-hop communications paths that allow operationsover greater ranges. Relay chains are particularly useful for missionsin urban terrain or to extend communications around corners, and intocave/tunnel complexes and bunkers, allowing access to more remote areasthat a single robot could not access.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a robot and remote control system.

FIGS. 2 a-d are perspective, front, side and top views of a robot havingflippers.

FIGS. 3 a-p are side views of a sequence of stair surmounting maneuvers.

FIG. 4 is flowchart of an obstacle surmounting control routine.

FIGS. 5 a-r are side views of another sequence of stair surmountingmaneuvers

FIG. 6 is flowchart of another obstacle surmounting control routine.

FIG. 7 is a perspective view of a robot in an elevated position during aself-righting maneuver.

FIG. 8 is a perspective view of a robot in a partially elevatedposition.

FIG. 9 is a partially exploded view of a robot chassis.

FIG. 10 is a perspective view of a flipper drive system.

FIG. 11 is a perspective view of opposing sprockets of a slip clutch.

FIG. 12 is a cross-sectional view of the opposing sprockets of the slipclutch assembled on a drive axle.

FIG. 13 is a perspective view of a robot including sensor zones

FIGS. 14 a-c are perspective and like views of an electrical subassemblyof a robot.

FIG. 15 is a functional block diagram of system components of a robot.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Various tracked robotic vehicles have been developed that are thesubject of, for example, U.S. Pat. Nos. 6,431,296, 6,263,989, 6,668,951and 6,615,885. These patents are instructive on the construction oftracked robotic vehicles having driven flippers, and means ofarticulation of robotic components, and are hereby incorporated byreference in their entirety into this application.

Autonomous control routines and control systems useful, for example, toavoid an obstacle, escape an area, or navigate an area, optimizecommunications or coverage or seek radio performance are disclosed inU.S. patent application Ser. No. 11/633,869 filed Dec. 4, 2006 andtitled “AUTONOMOUS COVERAGE NAVIGATION SYSTEM,” and U.S. patentapplication Ser. No., 12/100,782, filed Apr. 10, 2008 and titled“ROBOTICS SYSTEMS,” the entire contents of which are incorporated hereinin their entirety by reference. For example, to avoid an obstacle, acontrol routine can cause the robot to travel in an outward spiral or tobounce and recoil from obstacles making random turns.

With reference to FIG. 1, a mobile robotic vehicle or robot 2 isoperable via an Operator Control Unit (OCU) or remote control system 4.Remote communications can be transmitted via radio signal, infra redsignal, wi-fi signal, cellular signal, or other suitable signal. In somecircumstances, robot 2 can perform automated routines without remote useintervention. In other cases, a combination of automated routines anduser controls are used to maneuver robot 2. Remote control system 4 caninclude any number of radio transceivers or other communications linksand can interface with any number of robots or though any number ofrobots as mesh network nodes or other parts of a communications network.

This version of robot 2 is sized to be portable, and to substantiallyfit within a bounding volume approximately 18 cm (7 in.) long, 12 cm (5inches) wide and 6 cm (2 inches) tall. The overall dimensions of aparticular embodiment are about 16 cm×12 cm×6 cm excluding the antennaand camera protruding from the chassis, with a total mass of about 0.5kg and a top speed of about 0.6 m/s over a smooth surface. In someembodiments, the vehicle is configured to fit in a combat uniform cargopants pocket. Multiple robots 2 can be stacked to fit in a backpack.Compactness and portability allow robot 2 to be easily transported by anindividual and to be deployed into an area by simply tossing the robot.

The robot is removed from a carrying compartment, is activated bypulling a grenade-style pull pin, and is then tossed down a corridor, upa stairwell or into a window. The platform can be thrown by a singleperson or launched into an upper window or rooftop using an improvisedslingshot. Landing on a top floor, the remotely operated platform may beable to bypass some typical obstacles and travel down stairs. Theoperator can evaluate the resultant video before determining the nextcourse of action. The robot can also be used to search for and assessbooby traps, enemy personnel, and improvised explosive devices (IEDs).

In a particular embodiment, each of the robot 2 and remote controller 4and weight less than eight pounds. In some cases the robot is between0.5 and 1.0 Kg and the remote control 4 is a small PDA with a mass lessthan 0.5 kg. A separate charging station can be used for both the remotecontrol 4 and robot 2.

Remote control system 4 allows an operator to control robot 2 from adistance. The operator can select different levels of human control overthe robot, ranging from a teleoperation mode, in which the operatordirectly controls the motors and actuators on the robot, to autonomousoperation, in which the operator passes higher-level command to therobot. In partially autonomous operation, robot 2 can perform tasks suchas following a wall, avoiding an obstacle, surmounting an obstacle,avoiding a drop off or “cliff,” avoiding becoming high centered, evadinga moving object, positioning a transceiver, self-righting, repositioningto optimize communication network coverage, and the like.

Alternative versions of the remote control system 4 supportteleoperation as well as a means of switching between teleoperation andautonomous control. The user can interrupt autonomous operation of therobot at any time to give commands and direction, and the robot canoperate autonomously when not receiving particular directions from theuser. The system provides predetermined warning signals to the operator,for instance if it is unable to operate autonomously, possibly by meansof a vibrating unit that could be worn by the operator and which wouldbe effective in a noisy environment. In addition, the user can addadditional tasks to the robot's mission and request notification fromthe robot when milestone tasks have been achieved.

Versions of the robot can perform various autonomous tasks which can beinitiated by the operator from remote control system 4. These includeobstacle avoidance, wall following, climbing stairs, recovery from highcentering, self-righting, returning “home,” searching for a designatedobject, mapping, and establishing a communications network. The robotcan use the various mobility modes described above in these autonomousoperations, and if necessary, can call for operator assistance duringits execution of a task. Alternative configurations of pivotal arms canbe used. For example, a single central “arm” can be used.

With reference to FIGS. 2 a-d, robot 2 includes a body portion orchassis 6 housing, inter alia, drive components, a power supply, controlsystem and communication module. A pair of drive wheels 8 is positionedat a forward end of chassis 6 and a pair of idler wheels 10 ispositioned at a rearward end of chassis 6. Wheels 8 and 10 can includespiral spokes to provide additional impact resistance.

A pair of resilient tracks 12 is trained about wheels 8 and 10 onopposing sides of chassis 6 extending from the sides of the chassis toprovide a driven support surface for robot 2. Tracks 12 are continuousflexible belts with interior surface features for engaging drive wheels8 and exterior surface features for gaining traction over variousterrains.

First and second rotatable arms or flippers 14 extend radially from acommon drive axle 16. In the illustrated embodiment, idle wheels 10 freespin about flipper drive axle 16. Flippers 14 are configured to extendthrough a 360 degree range of motion to allow robot 2 to perform variouspositioning, obstacle surmounting and self-righting maneuvers. In otherembodiments, a single rotatable arm can accomplish these functions. In aparticular embodiment, flippers 14 are made from a flame rated advancedformula polymer with 85D Shore hardness available from Quantum Cast,part number AFP3100FR, UL 94 FR and FAR 25.853. Through holes or otherfeatures can be provided on flippers 14 for attachment of flipperaccessories, e.g., cameras, sensors, or wheels positioned at the flippertip or along the flipper length. For example, a camera, antenna orsensor can be mounted on the end of the arm to provide better exposureor higher vantage point.

Flippers 14 can be rotated to any desired angle relative to chassis 6.Robot 2 is designed to move about in a variety of environments,including an urban environment of buildings (including staircases),streets, underground tunnels, as well as in vegetation, such as throughgrass and around trees. Robot 2 has a variety of features which providerobust operation in these environments, including impact resistance andtolerance of debris entrainment. Wheels 8 and 10 are positioned onchassis 6 to provide greater ground clearance when right side up, yetcan provide sufficient clearance in some embodiments for operation whenrobot 2 is inverted. In the present embodiments, robot 2 preferablyrecovers from a tumble or fall in which it is inverted by aself-righting function.

Chassis 6 and other rigid robot components are designed for strength andlow weight and are made from durable plastic, polymer, composites,7075-T6 aluminum or other suitable lightweight, impact resistantmaterials. Tracks 12, wheels 8 and 10 and flippers 14 are alsoconfigured to be impact resistant. For example, wheels 8 and 10 can be apliable material and can include spiraled spokes to provide a degree ofresilience. Impact resistance is accomplished, in part, by surroundingmuch of the vehicle with compliant tracks 12 with pliable cleats. Tracks12 and cleats provide a first layer of impact protection.

Tracks 12 are configured to provide skid steering and include compliantbelts made of polyurethane or a similar flexible material. The belts areabrasion resistant and have high strength and minimal stretch due tointernal steel or fiber cording. Tracks 12 define a left-rightalternating tread to smooth successive impacts on most surfaces with aspacing between successive edges on the sides to catch larger terrainfeatures for traction. Without a bogey rail, the robot tends to tread onthe portion of the tracks at wheels' bottom dead center.

Tracks 12 can be stretched over wheels 8 and 10 and driven primarily byfriction. The surfaces of wheels 8 and 10 contacting tracks 12 can beprovided with a fine knurl pattern to enhance friction with tracks 12and prevent slippage from formation of water films between wheels 8 and10 and tracks 12. Alternatively, tacks 12 and drive wheels 8 can beformed with complementary features to provide positive drive engagement.For example, wheels 8 and 10 can have V-shaped grooves around theircircumference to receive an integral V-shaped rib on the inside of track12.

Alternative embodiments of the robot can use other types of tracks, suchas tracks made up of discrete elements. However, debris may be caughtbetween elements and such tracks are generally heavier than flexiblebelts. Other flexible materials can also be used for continuous belttracks. Tracks 12 can include cleats, ridges, or other projections foradditional traction. Such cleats can be angled to divert debris awayfrom chassis 6.

Flippers 14 can be can be continuously rotated around axle 16. Flippers14 can be rotated to a forward “stowed” position next to chassis 6.Alternatively, flippers 14 can be rotated to a rearward trailingposition to prevent catching of the ends of flippers 14 on terrain, forexample in tall grass. In some embodiments, to prevent possible damage,flippers 14 can automatically return to a stowed position when robot 2detects that it is in free fall.

With reference to FIGS. 3 a-p and FIG. 4, flippers 14 are furtherconfigured to be driven to rotate at predetermined intervals in stair orobstacle surmounting maneuvers. Flipper positioning angles are statedwith reference to the horizontal axis of chassis 6 as shown in FIG. 4with 0 degrees being the stowed flipper position, 90 degrees being avertical position, 180 being a trailing position and 270 being adownward position. An obstacle surmounting control routine is initiatedby detection of an obstacle or predetermined scenario or by operatorinput. In state 1 of the routine, shown in FIG. 3A, robot 2 approaches astair with flippers 14 is a forward stowed position substantiallyparallel to the ground at 0 degrees.

During obstacle surmounting maneuvers, a main obstacle surmountingcontrol routine 100 is run on a controller at 64 Hz while samplingaccelerometer data at 16 Hz and updating the flipper position at 16 Hz.Upon detection of the stair or other obstacle, the control routineenters state 2, in which flippers 14 are rotated upward and rearwardbetween approximately 45 and 90 degrees, as shown if FIG. 3B. In state 3shown in FIG. 3C, the forward end of robot 2 begins to ascend the frontface or riser of a stair. Once the robot has ascended to a predeterminedposition shown in FIG. 3C as detected by an accelerometer, e.g., betweenabout 15 and 45 degrees or a sensor reading of about 0.75 g, the routineenters state 4. Passage of a predetermined time since entering state 3,e.g., 3 seconds, can also trigger the fourth state.

In the fourth state, flippers 14 are rotated further counterclockwise orrearward between the positions shown in FIGS. 3D-3G, e.g., between about90-125 degrees, as tracks 12 are further driven such that the forwardend continues to ascends the stair riser and rearward end of robot 2approaches the stair riser. After a predetermined time, e.g., 1 second,and as robot 2 approaches a substantially vertically position shown inFIG. 3G, flippers 14 contact the underlying surface at approximately 125degrees and the routine enters a fifth state.

In the fifth state, flippers 14 are rotated quickly counterclockwise tolift the rearward end of robot 2 through the range shown in FIGS. 3G-M,e.g., between about 125 and 275 degrees, while tracks 12 are drivenuntil the center of gravity of robot 2 clears the nose of the stair asshown in FIG. 3N. Flippers 14 further serve to resist back sliding orany wheelie tendency as robot 2 clears the stair nose.

In an optional sixth state, if flippers reach about 275 degrees and theaccelerometer has not detected that the center of gravity of the robothas cleared the nose of the stair, flippers 14 are kicked backward fromabout 275 degrees, e.g., to less than about 235 degrees, in an effort totopple robot 2 forward from a possible teetering position.

Once the accelerometer detects that the center of gravity of robot 2 hascleared the nose of the stair, the routine enters a seventh state. Instate 7, robot 2 overcomes the stair and tips forward as the center ofgravity clears the stair nose as shown in FIG. 3M. Upon entering state7, or after a short time delay, e.g., 1 second, flippers 14 are rotatedclockwise towards a trailing position as shown in FIG. 3O. This preventsflippers 4 from catching on the surmounted obstacle as the robot isdriven forward.

Once the accelerometer detects that the robot has settled atop the stairor after a predetermined period of state 7, e.g., 1 second, an eighthroutine state causes flippers 14 to return to a default position, e.g.,substantially vertical, to prepare to surmount a second stair. Flipperposition is determined in the different states using a flipper positionsensor.

States 1-8 and the various maneuvers shown in FIGS. 3B-P are thenrepeated as needed to surmount successive stairs or other obstacles.

Another obstacle surmounting routine 200 is described with reference toFIGS. 5A-R and FIG. 6. When the front wheels encounter a vertical stairriser, there may not be sufficient ground friction to allow thewheel/tracks to climb the riser. This is particularly true in dusty orsandy environments where the floor friction can be substantially lessthan that of the cleaner stair riser. To assist the front wheels ininitiating climbing, flippers 14 are used to initially raise the forwardend of the robot 2. This can also be particularly helpful if a stairriser is angled outward.

In state 1, the robot advances forward towards the stair riser as shownin FIG. 5A. Tracks 12 are driven forward until the robot reaches theriser.

In state 2, flippers 14 are rotated to the stowed position, e.g., 0degrees in preparation for lifting the nose of the robot.

In state 3, flippers 14 are rotated “clockwise” downward such that thedistal ends of flippers 14 contact the underlying surface forward of thecenter of gravity of robot 2 as shown in FIG. 5B. Tracks 12 are advancedto ascend the stair riser.

In state 4, continued clockwise rotation of flippers 14 causes theforward end of robot 2 to raise up off the underlying surface at to apredetermined angle, e.g., about 15-45 degrees, as shown in FIG. 5C. Ifthe angle is not reached within a preset time, e.g., 1.5 seconds, theroutine advances to the next state.

The predetermined angle is selected to approximate the angle at whichthe frictional forces between the tracks and the floor and the tracksand the stair riser are sufficiently balanced to prevent back-sliding ofthe robot. Balance of the frictional forces between the track the riserand underlying surface enables the track to ascend the riser withoutcontinued clockwise rotation of flippers 14. The routine canperiodically test to see if the friction balance point has been achievedby slightly lifting the flippers and using the accelerometers to detectbacksliding. Once it is determined that the balance point has beenreached or passed, the flipper is no longer needed to raise the forwardend of the robot. If the accelerometer detects slippage or backslidingof the robot, previous states can be repeated as needed. In someembodiments, the track velocity is coordinated with the flipper motionsto help maintain traction and frictional balance.

In state 5, flippers 14 are rotated clockwise to an “upwards” positionas shown in FIGS. 5 E-F, e.g., to 90 degrees, to prevent the robot fromflipping over backwards as the wheels continue to climb. This movementis preferably performed without substantially shifting the center ofgravity or introducing disturbances that would upset the frictionalbalance.

It is advantageous for flippers 14 to be long enough to extend forwardof the center of gravity, yet short enough to not get caught under astair nose when later rotating counterclockwise, as shown in FIGS. 5D-Eto prepare to raise the rearward end of the robot. An estimated maximumflipper length is calculated by adding the wheel radius to the productof the length of the robot chassis and cosine of the angle at which thefrictional forces are sufficiently balanced to enable continued climbingby the tracks. This flipper length provides sufficient clearance forretraction of the flippers from a forward to a rearward position afterpartial ascend of a stair rise by the robot. Of course, flipper lengthcan be dictated by anticipated obstacle profiles including moreaggressive forward riser angles.

In state 6, tracks 12 are advanced to position the chassis substantiallyvertically against the stair riser with flippers rotatedcounterclockwise to a point adjacent to or contacting the ground asshown in FIGS. 5G-J.

In state 7, flippers 14 are extended, e.g., from about 100 degrees to175 degrees, while tracks 12 are driven at a “matched” velocity as shownin FIGS. 5J-O, or slightly faster as dictated by the geometry of theproblem, to allow the track to evenly surmount the stair nose as therobot is pushed upward by flippers 14. Matching of the track velocity tothe flipper rotation means that the tracks are advancing a distanceequal to the amount of extension provided by rotation of flippers 14.Velocity matching can also be used to maintain an angle of incline ofchassis 6 as the robot surmounts the stair nose.

In state 8, tracks 12 are driven while flippers 14 simply drag behind toprevent backsliding or wheelies as shown in FIG. 5N. Flippers 14 arepaused momentarily at the point of maximum extension, e.g., when theflipper tip is farthest away from the track/step corner, while thetracks continue to be driven forward. As the track cleats bounce overthe stair nose, the flipper tips will bounce and drag along the groundcloser to the step wall. The track speed can be varied to achieve adesire bouncing pattern.

In state 9, flippers 14 are again rotated counterclockwise to provide anextra extension to slightly level out the robot. This “over extension”of the flippers can also help tip the robot center of gravity over thestair nose.

In state 10, tracks 12 are driven quickly while flippers 14 are slowlyrotated clockwise back to the full extension point to climb onto the topof the step as shown in FIGS. 5P-R. In this position, the flippers cancatch the robot should it happen to back slide or tumble backwards. Oncethe center of mass of the robot extends forward of the stair nose, therobot falls forward on top of the step.

Once the vehicle tips forward of the step nose (as indicated by theaccelerometers showing the tilt angle going back to level) flippers 14are restored to a default driving or stowed position.

In another control routine, flippers 14 can be continuously rotated toovercome a high centered position. A high centered position can bedetected in multiple ways. For example, monitoring of video data,monitoring accelerometer data, comparing odometer and navigational data,GPS data discrepancies. Track motions can be coordinated with flippermotions to pull the vehicle forward, e.g., by driving the tracks whenthe flipper is in contact with the surface at the same rate that theflipper is expected to pull the vehicle forward. Flipper rotation ratescan depend on the expected or detected terrain, e.g., whether theflipper tips will penetrate the terrain surface. The effective flipperradius can be dynamically determined by signal processing theaccelerometer signals after repeated rotations of the flipper as afunction of flipper tip penetration into the underlying surface. Toprevent “digging in,” the tracks can be driven when the flippers are incontact with the underlying surface. The flipper rotational rate can beselected as a function of surface penetration and movement of the robotover the terrain and baseline data for behavior of the robot drivingover different terrains.

In the depicted embodiment, the flippers extended substantially thedistance between the drive wheel axle and the idler wheel axle. In somecases, the flipper length is selected to fit entirely within the lengthof the chassis and to extend forward of the robot center of gravity. Insome cases the flippers are at least as long as the idler wheel radius.The flippers or flipper length can be selected based on the dimensionsof anticipated obstacles.

With reference to FIG. 7, flippers 14 are configured to extend from axle16 centrally to a point beyond the center of gravity of robot 2. Thisallows robot 2 to be inverted or self-righted simply by rotation offlipper 14 through an arc of 90 degree beyond contact with an underlyingsurface.

Rotation past a vertical stability point causes the robot to fall overcompleting the inversion. Self-righting is often required after tumblingdown stairs or other inclines, or from atop other obstacles. Robot 2 candescend stairs forwards or backwards with flippers 14 in a stowedposition, driving tracks 12 either direction and tumbling or rolling toa resting position.

In some embodiments, robot 2 has more ground clearance in oneorientation than another. In some cases, a camera, antenna, sensor orrobot accessory may need to be reoriented upward if robot 2 lands upsidedown after a descent from an obstacle.

With reference to FIG. 8, flippers 14 can also be rotated to partiallyelevate the rearward end of robot 2. In some cases, flippers 14 can beused to raise or upend robot 2 to position a camera, antenna, sensor,munitions or the like at a desired height or angle or for helping topull robot 2 from a high centered position.

Flippers 14 can be repeatedly or continuously rotated in eitherdirection to provide a “swimming” motion to help propel robot 2 throughloose debris, gravel, sand and the like. Flippers 14 can raise the noseof the vehicle, to both help start a climb and to elevate a fixedcamera.

With reference to FIG. 9 robot chassis 6 houses, inter alia, wheel drivemotors 20 and 22 for powering drive wheels 8 and a flapper drive motor24. Chassis 6 also houses an electrical subassembly 26 and battery (notshown) positioned below electrical subassembly. The battery is asignificant portion of the total weight of robot 2 and is positionedsubstantially centrally front to back and towards the bottom of chassis6. A clamshell chassis body design allows provides sufficient volume forelectronics and mechanical drive mechanisms within a protective cover.

Flipper drive motor 24 is used to control the angle between flappers 14and chassis 6. Flipper drive motor 24 is coupled via a gear reductiontrain to axle 16. A slip clutch can be used to transfer output torquefrom flipper drive motor 24 to axle 16. A slip clutch can be adjustableto set a predetermined slip torque. Flippers 14 are connected via solidaxle 16 and an optic sensor on axle 16 provides for detection of theposition of flippers 14 regardless of clutch slippage. One clutchembodiment includes two beveled gears engaged with a spring, similar toa cordless drill clutch. Axle 16 passes through a central opening inidler wheels 10 and fixedly connects to flippers 14.

At the rear of the robot are two flippers 14 with the ability to rotate360° continuously to flip the robot over when inverted. The flippersalso assist the robot in climbing and negotiating small obstacles. Alsointegrated into the flipper mechanism is a slip clutch to protect thegearing in case of impact.

Drive motors 20, and 22 are 1 watt DC brushed motors. In other versionsof the robot, brushless motors can be used. Drive motors 20 and 22 turnoutput drive gears that attach to the wheels via integral splines.Output drive gears are retained via brass or Delrin bushings thatregister and align complementary portions of the chassis body. Drivemotors 20 and 24 are geared down 29:1 to drive wheels 8.

Steering is accomplished using differential speed of the tracks 14 oneither side of the robot by varying the speed of drive motors 20 and 22.The robot will, in principle, skid around the center of chassis 6approximately at the midpoint of the length of tracks allowing completeturning with the extremes of the robot staying within a 23 cm (9″)diameter circle.

In some cases, tracks 14 can be driven while flippers 14 maintain an endof robot 2 elevated above an underlying surface, for example toreposition an elevated antenna or camera. Other preprogrammed flipper orrobot positions can include fully extended, stowed, inclined, upright,and “wheelie.” In addition, robot 2 can perform several maneuversincluding self righting, stair climbing, and recovery from highcentering.

The chassis body can further serve to retain bushings for moving partsand as a mounting surface for an antenna, camera, microphone, sensorsand the like. Dust and moisture seals can be provided where axles orother components pass through the chassis body. For example, brassbushing securing at openings around chassis body 6 serve to support axle16 and the idler wheel axles. Chassis body 6 can also carry an antennaconnector base (e.g., standard SMA antenna connector).

With reference to FIG. 10, flippers 14 are rotated by drive motor 24(0.4 watt DC brushed motor) via gear train 26. Flipper drive motor 24 isgeared down 298:1 to axle 16 to provide a torque of approximately 400mNm (˜2× required to lift vehicle weight). Clutch can be adjusted toprovide up to 700 mNm of slip torque. A slip clutch prevents overloadingof flipper drive motor 24 and gearing, for example due to an impact onthe arms.

Flippers 14 can be stowed parallel to chassis 6 and tracks 12 when it isdeployed by tossing or dropping it through a window or door or when therobot tumbles. In some embodiments, a mechanical energy storage providesfor sudden release to move the flippers to allow the robot to perform asmall leap motion. An example energy storage system can be a spring,flywheel or other mechanical energy storage mechanism.

With reference to FIGS. 11 and 12, a slip clutch 70 is provided on axle16 between drive motor 24 and flippers 14. Slip clutch 70 includes afirst sprocket 72 carrying a series of drive teeth 74 on a first rotarysurface 76. First sprocket 72 is fixedly attached to axle 16. A secondsprocket 78 defines a series of slots 80 in a second rotary surface 82for receiving the drive teeth 74 of first sprocket 72. Drive teeth 74and slots 80 remain engaged so long as first and second rotary surfaces76 and 82 remain substantially in contact.

Under sufficient toque, the tapered surfaces of teeth 74 cam rotarysurfaces 76 and 82 apart allowing teeth 74 to slip one or more slots.Sprockets 72 and 78 are biased towards engagement via a spring 84retained on axle 16. Spring 84 provides an axial force to slip clutch 70to resist separation of surfaces 76 and 82. First sprocket 72 isconnected to axle 16 while second sprocket 78 spins freely about axle 16when disengaged from sprocket 72. Second sprocket 78 includes gear teethabout its circumference to engage drive motor 24.

During obstacle surmounting maneuvers, drive motor 24 turns secondsprocket 78 which in turn rotates first sprocket 72 and axle 16 torotate flippers 14.

Alternatively, a slip clutch can be formed of sufficiently pliablematerial to allow flexure of rotary surfaces 76 and 82 under sufficienttorque. Any number of frictional or cammed surfaces or other known typesof slip clutches can be substituted for slip clutch 70.

With reference to FIG. 13, robot 2 is provided with a pair of endsensors 28 and side sensors 30. Sensors 28 can be positioned on one orboth ends of robot 2 and sensors 30 can be positioned on one or moresides of robot 2. Sensors 28 and 30 include IR emitter/detector pairs.Sensors 28 are directed substantially parallel in front of tracks 12 toact as cliff detectors to detect and avoid falls and sensors 30 aredirected outward from chassis 6 to act as wall detectors. Sensors 28 and30 can include filtering features to accommodate ambient sunlight.Sensors 28 and 30 provide feedback that is used by robot 2, for example,to follow a wall or avoid a drop off. Sensors 28 and 30 can includesonar, infra red, proximity, impact or other sensor suitable to detectthe presence or absence of an object in the sensor range. Additionalsensor based autonomous robot behavior routines are disclosed in U.S.Pat. No. 6,883,201, titled “ROBOT OBSTACLE DETECTION SYSTEM” which isincorporated herein by reference in its entirety.

Additional autonomous behavior routines and control systems aredisclosed in U.S. Pat. No. 6,809,490 titled “METHOD AND SYSTEM FORMULTI-MODE COVERAGE FOR AN AUTONOMOUS ROBOT” and U.S. Pat. No. 7,459,871titled “DEBRIS SENSOR FOR CLEANING APPARATUS,” which are incorporatedherein by reference in their entirety. The routines include motioncontrol and coverage behaviors such as spiral coverage, cruising, bounceand recoil from an obstacle, wall following, self-alignment, and escapebehaviors as selected by an arbiter according to principles of behaviorbased robotics. Additional reactive controls and behavior routines areprovided for reacting to and concentrating on a point of interest in thecoverage space. Similar behaviors can be used to seek out a peak signalstrength peak or radio hot spots or to reposition a robot as a node in amesh network.

Sensors can be shielded within the track volume, within the protectiveshell of chassis 6 or positioned on the front and rearward ends of thevehicle. The top and bottom portions of chassis 6 can be fitted with anynumber of sensors, cameras, antennae, chemical sensors, bio-sensors,radiation sensors and the like.

Additional robot sensors provide input regarding flipper rotationposition, connector to a charging station, presence of a deactivationplug (pull pin). For example, robot 2 can be powered off if sensorsdetect that drive motors 20 or 22 have stalled of if the robot isotherwise stuck.

Chassis 6 also supports a camera 32 and antenna 34 to provide videotelemetry and other communications data. Camera 32 is depictedpositioned slightly rear of center, with the lens angled up to minimizethe field of view obstructed by the robot vehicle itself. Flippers 14can be rotated to raise the nose of the vehicle further if the cameraview is insufficiently high. To look over an edge, flippers 14 can beused to raise the rear of the vehicle to depress the camera view angle.

Transmission of video telemetry data or other sensor data from within abuilding can enable a small force to quickly and safely assess alocation or situation. For example, a camera can be used to quickly andsafely determine the presence and location of an adversary or explosivein a building

Robot 2 includes the capability of carrying a variety of accessories orsensors, including cameras, sonar sensors, infra-red detectors, inertialsensors, motor position, velocity and torque sensors, inclinometers, amagnetic compass, microphones, sound generator, or small weapon. Sensorscan be placed on all surfaces of the robot. For example, night time orlow light operation can be performed using onboard light such as aninfra-red (IR) array with a useful range of several meters. A smallwhite light can also be provided for up close color identification ofobjects.

A multi camera array can provide stereoscopic vision for navigation andvideo transmission back to remote control system 4. For example,multiple cell phone style cameras, each with multi-megapixel accuracyand a 90° field of view, to provide full 360° field of view. The robotcan be configured to monitor for motion and alert the operator if motionis detected. Similarly, an onboard microphone can enable an alert to besent to the operator if sound above a designated threshold is detected.

Onboard computing coupled with a multi megapixel imager can provide highresolution image capture and digital pan tilt zoom of the digitallycompressed and encrypted video stream. This minimizes the mechanicalcomplexity of the system by eliminating the need for a mechanicalpan-tilt assembly, and allows the use of image processing for unattendedoperation such as change detection and digital video recording ofmotion. Integrated infrared illuminators can provide sufficientillumination for navigation in an urban environment, while white lightilluminators can be used to identify targets up close. One example is a1.3 Megapixel camera with mpeg4 compression capabilities.

With reference to FIGS. 14 a-b, electrical subassembly 26 is mountedbetween the flipper drive motor 24 and wheel drive motors 20, 22 abovebatteries 52. Electrical subassembly includes a main printed circuitboard (PCB) 40 to which are electrically connected a removable massmemory 44, USB communication module 46; SDIO communication module 48,SDIO port 50. The battery, drive motors 20, 22 and 24, camera 32 andantenna 34 are operably coupled to electrical subassembly 26. PCB 40also carries end sensors 28 and side sensors 30. PCB 40 can includerigid circuit boards, flexible polyimide circuits, or other circuitmodules or combinations thereof, and may provide power regulation,motion control, sensors, and other functions.

Battery 52 includes a lithium ion battery pack with three 18650 cells inseries. Each cell has a capacity of 2.6 AHr, and contains 0.78grams/cell of lithium, or 2.3 g of lithium per assembled robot. Internalrechargeable Li-Ion battery pack 52 has a two hour charge time via 110 vor 220 v circuits. Both robot 2 and the OCU remote control 4 can becharged by a single adapter capable of accepting universal power(100-240 VAC 50/60 Hz). Optional charge adapters can be used forcharging at 12-24 VDC. Battery 52 can be attached directly to PCB 40 viaVHB tape. Solar power can be used to charge the battery or provide forextended duration low power surveillance.

A substantial capacitor bank is used to minimize the ripple in thebattery draw in powering the drive motors. It may be desirable in somecases to destroy internal circuits by reversing the polarity of thecapacitor bank into the lithium batteries to ignite the batteries. Asudden reversal of the energy from the capacitors creates a largecurrent surge sufficient to cause an electrical fire. This would helpfrustrate the ability of hostile warfighters from re-using any of thecomponents.

PCB 40 includes one or more computer processors and associated memorysystems. PCB 40 is coupled to communication modules 46, 48, whichinclude, for example, a radio for exchanging control and feedbackinformation with remote control system 4. Communications range with USBand SDIO radios was experimentally found to be approximately 40 metersof open area or through two cinderblock walls of a building.

Odometry sensors detect a pattern referenced to axle 16, such as aslotted or patterned strip secured to an axle, e.g., via a piece ofclear heatshrink tubing, or a slotted disc attached to an axle or drivenwheel 8. The odometery sensor is located on the idler wheel to accountfor track slippage on drive wheel 8. Odometry reading accuracy may beincreased by harder turns as opposed to sweeping turns.

Additional sensors determine the angle between flippers 14 and chassis 6and the rate of rotation of flippers 14 or wheels 8 or 10. An angularrate sensor is placed near the center of gravity of the robot 2 to trackthe bearing of the robot and provide increased positioning accuracy,facilitating movement in areas with few visual landmarks. Optionalaccelerometers can be located near the angular rate sensor. These inputsare used during full or partial autonomous robot operation.

With reference to FIG. 15, a functional block diagram of electricallyconnected system components of an embodiment of a robot is shown. PCB 40is electrically connected to an iMX31 processor 54, USB communicationmodule 46, SDIO communication module 48, flash memory 44, infraredsensors 28, proximity sensors 30, STmicro LIS344ALH three axisaccelerometer 56, flipper rotary position sensor (RPS) 58, SDIO/USBPayload PCB 42, and STmicro LISY300AL angular right sensor, camera 32,battery 52, drive motors 20, 22 and 24 with appropriate interfaces,controllers and the like.

The electrical components may also include one or more of the following:microphone, yaw sensor, active/passive analog IR LED andphototransistors, SDIO radios, 802.11b/g/n radio, satellite phone, EVDOcellular phone, USB peripherals, additional batteries, Bluetechnix IMX,GPS transponders and the like.

In various embodiments, communications modules 46 and 48 serve toprovide multi-hop style communications chains, to extend the usefulnessof the robots deep into radio frequency (RF) denied areas using standardOptimized Link State Routing daemon (OLSRd) software mesh networking.The robots can be repositioned to maintain a self healing communicationsnetwork

According to one method of establishing a mesh network, the deactivationplug is removed to activate each robot. The robots are then placed inapproximate locations for autonomous mesh networking. The robotsautomatically reposition to maintain the mesh network.

Advantageous Mesh Network Capabilities are disclosed generally in theLANdroids Bidders Day Briefing, Document Number BAA 07-46, released Jul.6, 2007 and available from DARPA.

When the mission is over, the robots can be recovered as required andcharged to be ready for the next mission. When the robots are not in useand not being charged, the deactivation plugs are installed into thecharge connectors to power off the robots and keep the batteries fromdraining prior to the next mission.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, alternative embodiments can include four or six driven wheelsand a single or multiple trailing pivoting arms. Accordingly, otherembodiments are within the scope of the following claims.

1. A method performed by a mobile robotic vehicle for climbing a stair,the method comprising: driving a support surface of the vehicle over anunderlying surface towards the stair; pivoting a trailing arm downwardagainst the underlying surface and causing a forward end of the vehicleto raise up off the underlying surface, the trailing arm having a distalend that contacts the underlying surface forward of a center of gravityof the vehicle; further driving the support surface to cause the forwardend of the vehicle to ascend a riser of the stair, the support surfacegenerating sufficient traction against the riser to climb the riser asthe support surface is driven; and pivoting the trailing arm so that thedistal end contacts the underlying surface at a point behind the vehiclewhile the vehicle ascends the stair.
 2. The method of claim 1, whereinpivoting the trailing arm so that the distal end contacts the underlyingsurface at a point behind the vehicle comprises pivoting the trailingarm to raise a rearward end of the vehicle while the forward end of thevehicle is supported by the first stair.
 3. The method of claim 1,wherein pivoting the trailing arm so that the distal end contacts theunderlying surface at a point behind the vehicle comprises: pivoting thetrailing arm to raise a rearward end of the vehicle while the forwardend of the vehicle is supported by the first stair; driving the supportsurface to advance the forward end of the robot over an uppermost edgeof the first stair riser; and pivoting the arm to further raise therearward end of the vehicle such that the forward end of the vehicletips downward beyond the uppermost edge of the riser of the first stair.4. The method of claim 3, wherein pivoting the trailing arm to raise therearward end of the vehicle while the forward end of the vehicle issupported by the first stair comprises pivoting the trailing arm at afirst rotational speed, and then pivoting the trailing arm at a secondrotational speed greater than the first rotational speed.
 5. The methodof claim 1, further comprising repeating the method to climb anotherstair.
 6. The method of claim 1, wherein: pivoting a trailing armdownward against the underlying surface and causing a forward end of thevehicle to raise up off the underlying surface comprises pivoting thetrailing arm in a clockwise direction; and pivoting the trailing arm sothat the distal end contacts the underlying surface at a point behindthe vehicle comprises pivoting the trailing arm in a counter-clockwisedirection.
 7. The method of claim 1, further comprising pivoting thetrailing arm to an upwards position perpendicular to the vehicle priorto pivoting the trailing arm so that the distal end contacts theunderlying surface at a point behind the vehicle.
 8. The method of claim7, wherein pivoting the trailing arm to an upwards positionperpendicular to the vehicle does not substantially shift the center ofgravity of the vehicle.
 9. The method of claim 1, wherein pivoting thetrailing arm downward against the underlying surface raises the vehicleto a predetermined angle of incline.
 10. The method of claim 9, whereinthe predetermined angle of incline is an angle at which frictionalforces between the support surface and the underlying surface andbetween the support surface and the stair are sufficiently balanced toprevent backsliding of the vehicle.
 11. The method of claim 1, whereinfurther driving the support surface to cause the forward end of thevehicle to contact and ascend a riser of the stair comprises lifting thetrailing arm and using an accelerometer to detect backsliding.
 12. Themethod of claim 11, further comprising pivoting the trailing arm toadjust an angle of incline of the vehicle in response to detectingbacksliding.
 13. The method of claim 11, further comprising pivoting thetrailing arm away from the underlying surface in response to detectingno backsliding.